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ERV FAQ: Of Eloi and Morlocks

From The Time Machine 1960, directed by George Pal, based on the 1895 novel by H.G. Wells.

Adapted from Wikipedia: "By the year AD 802,701, humanity has evolved into two separate species: the Eloi and the Morlocks. The Eloi live a life of ease on the surface, while the Morlocks live underground, tending machinery and providing for the Eloi. Having nothing they need to do, the Eloi have slowly become like cattle, smaller than modern humans with sub-human intelligence. All they do is feed, play, and mate. When one of them, Weena, falls into a river, none of the other Eloi help her. She is rescued instead by the Time Traveler. Every so often the Morlocks capture individual Eloi for food, and because this typically happens on moonless nights, the Eloi are terrified of darkness."

How would we prove that the Eloi and the Morlocks had indeed both descended from modern humans? Well, we have signatures in our DNA that were written by retroviruses. 

They put their DNA into the DNA of our cells to "fool" them into making new retroviruses. 

Sometimes, a retrovirus puts its DNA into the DNA of an egg or sperm cell. When this happens, it becomes heritable - you can inherit it from your parents and pass it down to your offspring. These heritable bits of viral DNA are called ERVs - Endogenous RetroViruses. Fortunately, they no longer fully perform their original functions, otherwise, humans would be extinct.

The signature of any endogenous retrovirus is that it can be found in exactly the same place in the DNA of every single one of your nuclear cells (every cell with a nucleus - which is most of them). The viral DNA ends up in the same place because it, along with all the rest of your DNA, is copied from the DNA of the original single cell you started out as. In contrast, the viral DNA that you have acquired from viruses in the environment is not present in all of your nuclear cells, and in those where it is present, the viral DNA ends up in different points in your DNA, comparing one infected cell with another.

To recap:

Viral DNA in the same place in the DNA of every cell means that it is inherited.
Viral DNA in different places in only some of your cells means that you have caught a virus from the environment.

Now if we find that all the Eloi have the same viral DNA as us, in every cell, in exactly the same places as us, we know that they must have inherited it from us, and therefore we are their ancestors.

The same goes for the Morlocks. The same viral DNA in the same places in every cell means that the Morlocks also descend from us.

Note that we do not even need to look at our own DNA! If we find the same viral DNA in the same places in every cell of every Eloi and Morlock, they have to have had common ancestors from which they inherited it (us!).

Bear this in mind when the ERV FAQ mentions, not Eloi and Morlocks, but humans and chimpanzees!